U.S. patent application number 15/735819 was filed with the patent office on 2020-02-13 for nonaqueous electrolyte secondary batteries.
This patent application is currently assigned to Panasonic Intellectual Property Management Co., Ltd.. The applicant listed for this patent is Panasonic Intellectual Property Management Co., Ltd.. Invention is credited to Daizo Jito, Akihiro Kawakita, Takeshi Ogasawara.
Application Number | 20200052289 15/735819 |
Document ID | / |
Family ID | 57942673 |
Filed Date | 2020-02-13 |
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United States Patent
Application |
20200052289 |
Kind Code |
A1 |
Jito; Daizo ; et
al. |
February 13, 2020 |
NONAQUEOUS ELECTROLYTE SECONDARY BATTERIES
Abstract
An object of the present invention is to provide a nonaqueous
electrolyte secondary battery that can attain a smaller increase in
direct current resistance after charge discharge cycles. An aspect
of the invention resides in a nonaqueous electrolyte secondary
battery wherein a positive electrode active material includes a
secondary particle formed by aggregation of primary particles of a
lithium transition metal oxide, and a secondary particle formed by
aggregation of primary particles of a rare earth compound. On a
surface of the secondary particle of the lithium transition metal
oxide, the secondary particle of the rare earth compound is
attached to a recess formed between adjacent primary particles of
the lithium transition metal oxide in such a manner that the
secondary particle of the rare earth compound is attached to each
of the primary particles forming the recess. The lithium transition
metal oxide includes magnesium dissolved therein.
Inventors: |
Jito; Daizo; (Osaka, JP)
; Kawakita; Akihiro; (Hyogo, JP) ; Ogasawara;
Takeshi; (Hyogo, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Panasonic Intellectual Property Management Co., Ltd. |
Osaka-shi, Osaka |
|
JP |
|
|
Assignee: |
Panasonic Intellectual Property
Management Co., Ltd.
Osaka-shi, Osaka
JP
|
Family ID: |
57942673 |
Appl. No.: |
15/735819 |
Filed: |
July 28, 2016 |
PCT Filed: |
July 28, 2016 |
PCT NO: |
PCT/JP2016/003499 |
371 Date: |
December 12, 2017 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01M 4/131 20130101;
H01M 4/525 20130101; H01M 4/62 20130101; H01M 4/364 20130101; H01M
4/485 20130101 |
International
Class: |
H01M 4/36 20060101
H01M004/36; H01M 4/485 20060101 H01M004/485; H01M 4/525 20060101
H01M004/525 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 6, 2015 |
JP |
2015-156220 |
Claims
1-7. (canceled)
8. A nonaqueous electrolyte secondary battery comprising a positive
electrode, a negative electrode and a nonaqueous electrolyte,
wherein the positive electrode comprises a positive electrode
active material that comprises: a secondary particle formed by
aggregation of primary particles of a lithium transition metal
oxide, and a secondary particle formed by aggregation of primary
particles of a rare earth compound; on a surface of the secondary
particle of the lithium transition metal oxide, the secondary
particle of the rare earth compound is attached to a recess formed
between adjacent primary particles of the lithium transition metal
oxide in such a manner that the secondary particle of the rare
earth compound is attached to each of the primary particles forming
the recess; the lithium transition metal oxide includes magnesium
dissolved therein; and the concentration of magnesium dissolved in
a region that extends from the surface of the secondary particle of
the lithium transition metal oxide to 20% of the particle size of
the particle is higher than the concentration of magnesium
dissolved in the particle except the region.
9. The nonaqueous electrolyte secondary battery according to claim
8, wherein the secondary particle of the rare earth compound is
attached to both of the adjacent primary particles forming the
recess.
10. The nonaqueous electrolyte secondary battery according to claim
8, wherein the concentration of magnesium dissolved in the lithium
transition metal oxide is not less than 0.03 mol % and not more
than 0.5 mol % relative to the total molar amount of metal
element(s) except lithium.
11. The nonaqueous electrolyte secondary battery according to claim
8, wherein the concentration of magnesium dissolved in the region
is not less than 0.03 mol % and not more than 0.5 mol % relative to
the total molar amount of metal element(s) except lithium.
12. The nonaqueous electrolyte secondary battery according to claim
8, wherein an average particle size of the secondary particle of
the rare earth compound is not less than 100 nm and not more than
400 nm.
13. The nonaqueous electrolyte secondary battery according to claim
8, wherein an average particle size of the secondary particle of
the rare earth compound is not less than 150 nm and not more than
300 nm.
14. The nonaqueous electrolyte secondary battery according to claim
8, wherein at least one rare earth element selected from neodymium,
samarium and erbium constitutes the rare earth compound.
15. The nonaqueous electrolyte secondary battery according to claim
8, wherein the proportion of nickel in the lithium transition metal
oxide is not less than 80 mol % relative to the total molar amount
of metal element(s) except lithium.
16. The nonaqueous electrolyte secondary battery according to claim
8, wherein the proportion of cobalt in the lithium transition metal
oxide is not more than 7 mol % relative to the total molar amount
of metal element(s) except lithium.
Description
TECHNICAL FIELD
[0001] The present disclosure relates to nonaqueous electrolyte
secondary batteries.
BACKGROUND ART
[0002] In recent years, nonaqueous electrolyte secondary batteries
are required to have an increased capacity so that they can be used
for a long period of time, and are also required to be enhanced in
output characteristics so that they can be charged and discharged
repeatedly at a large current in a relatively short time.
[0003] For example, Patent Literature 1 suggests that a Group III
element in the periodic table that is present on the surface of
base particles as a positive electrode active material can restrain
reaction between the positive electrode active material and an
electrolytic solution from occurring even when the charge voltage
is increased, and can reduce a deterioration in charge storage
characteristics.
[0004] Patent Literature 2 suggests that dissolving magnesium (Mg)
into a positive electrode active material decreases the
crystallinity of the positive electrode and thus can improve
discharge performance.
CITATION LIST
Patent Literature
[0005] PTL 1: WO 2005/008812
[0006] PTL 2: WO 2014/097569
SUMMARY OF INVENTION
Technical Problem
[0007] Unfortunately, it has been found that batteries, even with
the techniques disclosed in Patent Literatures 1 and 2, suffer an
increase in direct current resistance (hereinafter, sometimes
written as DCR), in other words, are deteriorated in output
characteristics, after being subjected to charge discharge
cycles.
[0008] It is therefore an object of the present disclosure to
provide a nonaqueous electrolyte secondary battery that can attain
a smaller increase in DCR after charge discharge cycles.
Solution to Problem
[0009] A nonaqueous electrolyte secondary battery according to the
present disclosure includes a positive electrode, a negative
electrode and a nonaqueous electrolyte, wherein the positive
electrode includes a positive electrode active material that
includes a secondary particle formed by aggregation of primary
particles of a lithium transition metal oxide, and a secondary
particle formed by aggregation of primary particles of a rare earth
compound. On a surface of the secondary particle of the lithium
transition metal oxide, the secondary particle of the rare earth
compound is attached to a recess formed between adjacent primary
particles of the lithium transition metal oxide in such a manner
that the secondary particle of the rare earth compound is attached
to each of the primary particles forming the recess. The lithium
transition metal oxide includes magnesium dissolved therein.
Advantageous Effects of Invention
[0010] The nonaqueous electrolyte secondary battery according to
the present disclosure can attain a smaller increase in DCR after
charge discharge cycles.
BRIEF DESCRIPTION OF DRAWINGS
[0011] FIG. 1 is a front view of a nonaqueous electrolyte secondary
battery according to an example embodiment.
[0012] FIG. 2 is a sectional view along line A-A in FIG. 1.
[0013] FIG. 3 is a partially enlarged sectional view of a positive
electrode active material particle according to an example
embodiment.
[0014] FIG. 4 is an enlarged sectional view of part of a
conventional positive electrode active material particle.
[0015] FIG. 5 is an enlarged sectional view of part of a
conventional positive electrode active material particle.
DESCRIPTION OF EMBODIMENTS
[0016] Example embodiments will be described in detail below with
reference to the drawings.
[0017] The present disclosure is not limited to the embodiments
discussed herein, and various modifications are possible without
departing from the spirit of the present disclosure. The drawings
used in the description of the embodiments are only
illustrative.
[0018] FIG. 1 is a view illustrating a nonaqueous electrolyte
secondary battery 11 according to an example embodiment.
[0019] As illustrated in FIG. 1 and FIG. 2, the nonaqueous
electrolyte secondary battery 11 includes a positive electrode 1, a
negative electrode 2 and a nonaqueous electrolyte (not shown). The
positive electrode 1 and the negative electrode 2 are wound via a
separator 3 so as to form, together with the separator 3, a flat
electrode assembly. The nonaqueous electrolyte secondary battery 11
includes a positive electrode current collector tab 4, a negative
electrode current collector tab 5, and an aluminum laminate case 6
which has a closed portion 7 formed by heat sealing of peripheral
regions. The flat electrode assembly and the nonaqueous electrolyte
are accommodated in the aluminum laminate case 6. The positive
electrode 1 is connected to the positive electrode current
collector tab 4, and the negative electrode 2 to the negative
electrode current collector tab 5. The structure thus formed is
chargeable and dischargeable as a secondary battery.
[0020] While the example shown in FIG. 1 and FIG. 2 illustrates a
laminate film pack battery including a flat electrode assembly, the
present disclosure may be applied to other types of batteries. The
shape of the battery may be, for example, cylindrical, prismatic,
coin shape, or the like.
[0021] Hereinbelow, constituents of the nonaqueous electrolyte
secondary battery 11 will be described in detail.
[Positive Electrodes]
[0022] For example, the positive electrode includes a positive
electrode current collector such as a metallic foil, and a positive
electrode active material layer disposed on the positive electrode
current collector. The positive electrode current collector may be,
for example, a foil of a metal that is stable at positive electrode
potentials such as aluminum, or a film having a skin layer of such
a metal. The positive electrode mixture layer includes a positive
electrode active material, and preferably further includes a
conductive agent and a binder. The positive electrode may be
fabricated by, for example, applying a positive electrode mixture
slurry including the positive electrode active material and other
components such as a conductive agent and a binder onto a positive
electrode current collector, and drying and rolling the wet films
so as to form positive electrode mixture layers on both sides of
the current collector.
[0023] The conductive agent may be used to enhance the electrical
conductivity of the positive electrode active material layers.
Examples of the conductive agents include carbon materials such as
carbon black, acetylene black, Ketjen black and graphite. These may
be used singly, or two or more may be used in combination.
[0024] The binder may be used to enhance the bonding of components
such as the positive electrode active material with respect to the
surface of the positive electrode current collector while ensuring
a good contact between the positive electrode active material and
the conductive agent. Examples of the binders include fluororesins
such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride
(PVdF), polyacrylonitrile (PAN), polyimide resins, acrylic resins
and polyolefin resins. These resins may be used in combination with
carboxymethylcellulose (CMC) or salts thereof (such as CMC-Na,
CMC-K and CMC-NH.sub.4, and partially neutralized salts),
polyethylene oxide (PEO) and the like. These may be used singly, or
two or more may be used in combination.
[0025] Hereinbelow, positive electrode active material particles
according to an example embodiment will be described in detail with
reference to FIG. 3.
[0026] FIG. 3 is a partially enlarged sectional view of a positive
electrode active material particle according to an example
embodiment.
[0027] As illustrated in FIG. 3, the positive electrode active
material particle includes a secondary particle 21 of a lithium
transition metal oxide formed by aggregation of primary particles
20 of a lithium transition metal oxide, and a secondary particle 25
of a rare earth compound formed by aggregation of primary particles
24 of a rare earth compound. On the surface of the secondary
particle 21 of the lithium transition metal oxide, the secondary
particle 25 of the rare earth compound is attached to a recess 23
between adjacent primary particles 20 of the lithium transition
metal oxide in such a manner that the secondary particle 25 of the
rare earth compound is attached to each of the primary particles 20
forming the recess 23. Further, the lithium transition metal oxide
that constitutes the positive electrode active material particles
includes magnesium (Mg) dissolved therein. The concentration of Mg
dissolved in the lithium transition metal oxide is preferably not
less than 0.03 mol % and not more than 0.5 mol % relative to the
total molar amount of metal element(s) except lithium.
[0028] Here, the phrase that the secondary particle 25 of the rare
earth compound is attached to each of the primary particles 20 of
the lithium transition metal oxide forming the recess 23 means that
the secondary particle 25 is attached to the surface of at least
two primary particles 20 that are adjacent to one another in the
recess 23. For example, the positive electrode active material
particles of the present embodiment are such that in a cross
section of the particle of the lithium transition metal oxide, the
secondary particle 25 of the rare earth compound is attached to the
surface of both of two primary particles 20 that are adjacent to
each other on the surface of the secondary particle 21 of the
lithium transition metal oxide. While some of the secondary
particles 25 of the rare earth compound may be attached to the
surface of the secondary particle 21 other than in the recesses 23,
most of the secondary particles 25, for example, not less than 80%
or not less than 90%, or substantially 100% of the secondary
particles 25 are present in the recesses 23.
[0029] In the positive electrode active material particles of the
present embodiment, the secondary particles 25 of the rare earth
compound that are each attached to both primary particles 20 of the
lithium transition metal oxide adjacent to each other suppress
surface alteration of the primary particles 20 during charge
discharge cycles, with the result that a breakage of the positive
electrode active material particles is prevented. In addition, the
secondary particles 25 of the rare earth compound probably have an
effect of fixing (bonding) adjacent primary particles 20 to one
another, and consequently the occurrence of breakage at interfaces
of the primary particles in the recesses 23 is suppressed even when
the positive electrode active material is repeatedly swollen and
shrunk during charge discharge cycles.
[0030] Further, Mg dissolved in the lithium transition metal oxide
makes it possible to suppress alteration and breakage at interfaces
of the primary particles 20 within the secondary particles 21.
Specifically, it is probable that the rare earth compound
suppresses deterioration at interfaces of the primary particles 20
on the surface of the secondary particles 21, and Mg suppresses
deterioration at interfaces of the primary particles 20 in the
inside of the secondary particles 21. As a result, the increase in
DCR after charge discharge cycles can be reduced and the decrease
in output characteristics can be rendered small.
[0031] The rare earth compound is preferably at least one compound
selected from hydroxides, oxyhydroxides, oxides, carbonate
compounds, phosphate compounds and fluoride compounds of rare
earths.
[0032] The rare earth element constituting the rare earth compound
is at least one selected from scandium, yttrium, lanthanum, cerium,
praseodymium, neodymium, samarium, europium, gadolinium, terbium,
dysprosium, holmium, erbium, thulium, ytterbium and lutetium. Of
these, neodymium, samarium and erbium are particularly preferable.
Compounds of neodymium, samarium and erbium are particularly
excellent in, for example, suppressive effects on surface
alteration that can occur on the surface of the secondary particles
21 (at interfaces of the primary particles 20) of the lithium
transition metal oxide, as compared to other rare earth
compounds.
[0033] Specific examples of the rare earth compounds include
hydroxides such as neodymium hydroxide, samarium hydroxide and
erbium hydroxide, oxyhydroxides such as neodymium oxyhydroxide,
samarium oxyhydroxide and erbium oxyhydroxide, phosphate compounds
such as neodymium phosphate, samarium phosphate and erbium
phosphate, carbonate compounds such as neodymium carbonate,
samarium carbonate and erbium carbonate, oxides such as neodymium
oxide, samarium oxide and erbium oxide, and fluoride compounds such
as neodymium fluoride, samarium fluoride and erbium fluoride.
[0034] The average particle size of the primary particles 24 of the
rare earth compound is preferably not less than 5 nm and not more
than 100 nm, and more preferably not less than 5 nm and not more
than 80 nm.
[0035] The average particle size of the secondary particles 25 of
the rare earth compound is preferably not less than 100 nm and not
more than 400 nm, and more preferably not less than 150 nm and not
more than 300 nm. If the average particle size of the secondary
particles 25 is excessively large, the number of recesses 23 in the
lithium transition metal oxide which can accept the secondary
particles 25 is decreased, with the result that the decrease in
capacity retention after high-temperature cycles cannot be rendered
sufficiently small at times. If, on the other hand, the average
particle size of the secondary particles 25 is excessively small,
the secondary particles 25 have a small area of contact with each
of the primary particles 20 in the recesses 23 in the lithium
transition metal oxide, and consequently may reduce their effect of
fixing (bonding) adjacent primary particles 20 and their effect of
suppressing the breakage on the surface of the secondary particles
21 of the lithium transition metal oxide.
[0036] The proportion (amount) in which the rare earth compound is
attached is preferably not less than 0.005 mass % and not more than
0.5 mass %, and more preferably not less than 0.05 mass % and not
more than 0.3 mass % in terms of rare earth element relative to the
total mass of the lithium transition metal oxide. If the proportion
is excessively low, the amount of the rare earth compound attached
to the recesses 23 in the lithium transition metal oxide is so
small that the rare earth compound may fail to attain the
aforementioned effects sufficiently. If, on the other hand, the
proportion is excessively high, the rare earth compound will cover
not only the recesses 23 but also the surface of the secondary
particles 21 of the lithium transition metal oxide, and may cause a
decrease in initial charge discharge characteristics.
[0037] The average particle size of the primary particles 20 of the
lithium transition metal oxide is preferably not less than 100 nm
and not more than 5 .mu.m, and more preferably not less than 300 nm
and not more than 2 .mu.m. If the average particle size of the
primary particles 20 is excessively small, too many interfaces of
the primary particles will be formed on and within the secondary
particles 21, and the primary particles may be easily broken by
swelling and shrinkage of the positive electrode active material
during charge discharge cycles. If, on the other hand, the average
particle size is excessively large, the amount of interfaces of the
primary particles on and within the secondary particles 21 is so
reduced that the output, particularly at low temperature, may be
reduced.
[0038] The average particle size of the secondary particles 21 of
the lithium transition metal oxide is preferably not less than 2
.mu.m and not more than 40 .mu.m, and more preferably not less than
4 .mu.m and not more than 20 .mu.m. If the average particle size of
the secondary particles 21 is excessively small, the packing
density of the positive electrode active material is decreased and
a sufficiently high capacity may not be attained at times. If, on
the other hand, the average particle size is excessively large, a
sufficient output may not be obtained, particularly at low
temperature. Because the secondary particles 21 are formed by the
primary particles 20 that are bonded (aggregated) together, there
are no primary particles 20 larger than the secondary particles
21.
[0039] The average particle size was determined by observing the
surface and cross sections of the active material particles with a
scanning electron microscope (SEM) and measuring the size of, for
example, several tens of particles for each type of particles. The
average particle size of the primary particles of the rare earth
compound means the size along the surface, not in the thickness
direction, of the active material.
[0040] The median particle size (D50) of the secondary particles 21
of the lithium transition metal oxide is preferably not less than 3
.mu.m and not more than 30 .mu.m, and more preferably not less than
5 .mu.m and not more than 20 .mu.m. The median particle size (D50)
may be measured by an optical diffraction scattering method. The
median particle size (D50) means the particle size at 50%
cumulative volume in the particle size distribution of the
secondary particles 21, and is also referred to as the
(volume-based) median diameter.
[0041] In the lithium transition metal oxide, the proportion of
nickel (Ni) in the oxide is preferably not less than 80 mol %
relative to the total molar amount of metal element(s) except
lithium (Li). For example, this configuration makes it possible to
increase the capacity of the positive electrode and facilitates the
occurrence of proton exchange reaction at the interfaces of the
primary particles 20 described later. The lithium transition metal
oxide preferably includes, in addition to nickel (Ni), at least one
of cobalt (Co), manganese (Mn) and aluminum (Al). Specific examples
of preferred lithium transition metal oxides include lithium nickel
manganese composite oxide, lithium nickel cobalt manganese
composite oxide, lithium nickel cobalt composite oxide, and lithium
nickel cobalt aluminum composite oxide. The lithium nickel cobalt
aluminum composite oxide may have a composition in which the
Ni:Co:Al molar ratio is, for example, 8:1:1, 82:15:3, 85:12:3,
87:10:3, 88:9:3, 88:10:2, 89:8:3, 90:7:3, 91:6:3, 91:7:2, 92:5:3 or
94:3:3. A single or a mixture of these oxides may be used.
[0042] When the lithium transition metal oxide has a Ni proportion
(a Ni content) of not less than 80 mol %, the proportion of
trivalent Ni is correspondingly high and consequently the proton
exchange reaction of lithium in the lithium transition metal oxide
with water is allowed to occur easily in water. A large amount of
LiOH generated by the proton exchange reaction comes out of the
inside to the surface of the particles of the lithium transition
metal oxide. As a result, the alkali (OH.sup.-) concentration
between the primary particles 20 of the lithium transition metal
oxide that are adjacent to one another on the surface of the
secondary particles 21 of the lithium transition metal oxide
becomes higher than its surrounding environment. The alkali present
in the recesses 23 between the primary particles 20 attracts the
primary particles 24 of the rare earth compound and facilitates
their attachment to the recesses while they are aggregated into the
secondary particles 25. When, in contrast, the lithium transition
metal oxide has a Ni proportion of less than 80 mol %, the
proportion of trivalent Ni is low and the proton exchange reaction
is difficult to occur, and consequently the alkali concentration
between the primary particles 20 is substantially the same as in
its surrounding environment. Thus, even if the primary particles 24
of the rare earth compound that have precipitated bond together
into secondary particles 25, such particles tend to be attached to
portions (elevated portions) of the surface of the lithium
transition metal oxide other than the recesses 23.
[0043] From points of view such as capacity enhancement, the
proportion of Co in the lithium transition metal oxide is
preferably not more than 7 mol %, and more preferably not more than
5 mol % relative to the total molar amount of metal element(s)
except Li. In the presence of scarce Co, a structural change occurs
more easily during charging and discharging and sometimes a
breakage occurs easily at particle interfaces. In view of this
fact, the suppressive effect on surface alteration is taken
advantage of more prominently.
[0044] As mentioned above, Mg is dissolved in the lithium
transition metal oxide. The concentration of Mg dissolved in the
lithium transition metal oxide is preferably not less than 0.03 mol
% and not more than 0.5 mol %, and more preferably not less than
0.05 mol % and not more than 0.3 mol % relative to the total molar
amount of metal element(s) except Li. If the amount of dissolved Mg
is excessively small, the element may fail to attain sufficiently
its effect of suppressing alteration and breakage at interfaces of
the primary particles 20 within the secondary particles 21. If, on
the other hand, the amount of dissolved Mg is excessively large,
the capacity per unit weight of the positive electrode active
material tends to be decreased. While details will be described
later, the presence or absence of dissolved Mg and the amount
(concentration) in which it is dissolved in the lithium transition
metal oxide may be determined by energy dispersive X-ray
spectrometry (EDS), inductively coupled plasma (ICP) emission
spectroscopy and SEM.
[0045] Mg in the lithium transition metal oxide may be dissolved
uniformly over the entirety of the secondary particles 21, but is
preferably enriched near the surface of the secondary particles 21.
That is, the secondary particles 21 preferably have a distribution
of the Mg concentration. By designing the lithium transition metal
oxide so that the concentration of Mg dissolved therein is higher
near the surface than near the core of the secondary particles 21,
it is possible to efficiently suppress alteration and breakage at
interfaces of the primary particles 20 near the surface of the
secondary particles 21 which have a larger influence on the
increase in DCR.
[0046] Specifically, it is preferable that the concentration of Mg
dissolved in a skin region that extends from the surface of the
secondary particle 21 of the lithium transition metal oxide to 20%
of the particle size of the particle be higher than the
concentration of Mg dissolved in the other region, namely, in the
particle except the skin region. Mg may be present only in the skin
region of the secondary particles 21 and may be substantially
absent in the other region of the secondary particles 21. The other
region of the secondary particles 21 is the region except the skin
region, and extends from the position corresponding to 20% of the
particle size to the core of the secondary particles 21
(hereinafter, the region is sometimes written as the "core
region"). Here, the particle size of the secondary particle 21 was
measured by circumscribing a circle on a particle imaged by SEM,
and measuring the diameter of the circumscribed circle.
[0047] The concentration of Mg dissolved in the skin region is
preferably not less than 0.03 mol % and not more than 0.5 mol %,
more preferably not less than 0.05 mol % and not more than 0.4 mol
%, and particularly preferably not less than 0.08 mol % and not
more than 0.35 mol % relative to the total molar amount of metal
element(s) except Li. When, for example, the amount (concentration)
of Mg dissolved in the entirety of the secondary particles 21 is
0.03 mol %, it is preferable that the concentration of Mg dissolved
in the skin region be above 0.03 mol %, and it is preferable that
the concentration becomes higher toward the surface of the
secondary particles 21. The concentration of Mg in the entirety of
the secondary particles 21 is preferably not less than 0.03 mol %
relative to the total molar amount of metal element(s) except Li.
If the concentration of Mg dissolved in the skin region is
excessively high, the amount of dissolved Mg is so large that the
initial charge discharge capacity is decreased at times.
[0048] The concentration of Mg dissolved in the skin region may be
measured (computed) by EDS and ICP emission spectroscopy. For
example, the concentration of Mg dissolved in the skin region of
the secondary particles 21 may be computed based on a Mg mapping
image of a cross section of the secondary particle 21 obtained by
EDS, and the Mg content determined by ICP emission
spectroscopy.
[0049] An example method for dissolving Mg into the lithium
transition metal oxide is to mix a magnesium compound with a
compound(s) of lithium, nickel and the like followed by calcination
(heat treatment) of the mixture, or to mix a magnesium compound
with a lithium transition metal oxide followed by calcination of
the mixture. By the former method, Mg can be dissolved uniformly in
the entirety of the secondary particles 21. By the latter method,
the concentration of Mg dissolved in the skin region of the
secondary particles 21 can be increased as compared to the
concentration of Mg dissolved in the core region. The calcination
is preferably performed at a temperature of 500 to 700.degree. C.,
and is carried out, for example, in an oxygen atmosphere or in the
air. The magnesium compound is not particularly limited and may be,
among others, magnesium hydroxide, magnesium oxide, magnesium
sulfate or magnesium nitrate.
[0050] For purposes such as to obtain batteries with excellent
high-temperature storage characteristics, the lithium transition
metal oxide is preferably washed with water or the like to remove
alkali components adhering to the surface of the lithium transition
metal oxide.
[0051] An example method for attaching the rare earth compound to
the surface of the secondary particles 21 of the lithium transition
metal oxide is to add an aqueous solution of the rare earth
compound to a suspension including the lithium transition metal
oxide. During the addition of an aqueous solution of the rare earth
compound to a suspension including the lithium transition metal
oxide, the pH of the suspension is desirably controlled to the
range of 11.5 and above, and preferably to the range of pH 12 and
above. The treatment under such conditions tends to cause the
particles of the rare earth compound to be unevenly distributed on
the surface of the secondary particles 21. When, on the other hand,
the pH of the suspension is controlled to the range of 6 to 10, the
particles of the rare earth compound tend to be attached uniformly
over the entire surface of the secondary particles 21. If the pH is
below 6, at least part of the lithium transition metal oxide may be
dissolved.
[0052] The pH of the suspension is desirably controlled to the
range of 11.5 to 14, and particularly preferably to the range of pH
12 to 13. If the pH is above 14, the primary particles 24 of the
rare earth compound may be excessively coarsened; further, an
excessively large amount of alkalis may remain inside the particles
of the lithium transition metal oxide to increase the risk of
gelation of a positive electrode mixture slurry during its
preparation, and may also adversely affect the storage stability of
batteries.
[0053] When the aqueous solution of the rare earth compound that is
added to the suspension including the lithium transition metal
oxide is a simple aqueous solution, the rare earth is precipitated
as the hydroxide. When the aqueous solution contains a sufficient
amount of carbon dioxide dissolved therein, the rare earth is
precipitated as the carbonate compound. When a sufficient amount of
phosphate ions are added to the suspension, the rare earth compound
that is precipitated on the surface of the lithium transition metal
oxide particles is the phosphate compound of the rare earth. By
controlling the types of ions dissolved in the suspension, for
example, a rare earth compound that is a mixture of hydroxide and
fluoride can be obtained.
[0054] The lithium transition metal oxide in which the rare earth
compound has been attached to the surface is preferably heat
treated. The heat treatment causes the rare earth compound to
strongly adhere to interfaces of the primary particles 20 so as to
attain enhancements in the suppressive effect on surface alteration
which can occur at interfaces of the primary particles 20 and in
the effect of bonding the primary particles 20 to one another, thus
facilitating obtaining excellent DCR suppressive effects.
[0055] The lithium transition metal oxide in which the rare earth
compound has been attached to the surface is preferably heat
treated in vacuum. The water derived from the suspension used to
attach the rare earth compound has penetrated to the inside of the
particles of the lithium transition metal oxide. Because the
secondary particles 25 of the rare earth compound have been
attached to the recesses 23 in the lithium transition metal oxide,
the inside water is inhibited from going out during drying. In view
of this, the heat treatment is preferably performed in vacuum so
that water can be removed efficiently. If the positive electrode
active material carries an increased amount of water when it is
installed into a battery, the water undergoes reaction with the
nonaqueous electrolyte to form a product which can alter the
quality of the surface of the active material.
[0056] The aqueous solution containing the rare earth compound may
be a solution of the compound in the form of, for example, acetate
salt, nitrate salt, sulfate salt, oxide or chloride in a
water-based solvent. When, in particular, a rare earth oxide is
used, the aqueous solution may be one which contains the sulfate
salt, chloride or nitrate salt of the rare earth obtained by
dissolving the oxide into an acid such as sulfuric acid,
hydrochloric acid or nitric acid.
[0057] If the rare earth compound is attached to the surface of the
secondary particles of the lithium transition metal oxide by a
method where the lithium transition metal oxide and the rare earth
compound are dry mixed, the particles of the rare earth compound
tend to be attached randomly to the surface of the secondary
particles of the lithium transition metal oxide. That is, it is
difficult to attach the rare earth compound selectively to the
recesses 23 in the lithium transition metal oxide. Further, the dry
mixing method encounters a difficulty in strongly attaching the
rare earth compound to the lithium transition metal oxide, and may
fail to attain sufficient effects in fixing (bonding) the primary
particles 20 to one another. Consequently, when, for example, the
positive electrode active material particles are mixed together
with components such as a conductive agent and a binder to give a
positive electrode mixture, the rare earth compound may be detached
easily from the lithium transition metal oxide.
[0058] The positive electrode active material is not limited to the
above particles of the lithium transition metal oxide alone. The
lithium transition metal oxide described above may be used as a
mixture with other positive electrode active materials. Such
additional positive electrode active materials are not particularly
limited as long as the compounds allow lithium ions to be inserted
therein and released therefrom reversibly. Examples thereof include
active materials that allow lithium ions to be intercalated and
deintercalated while maintaining a stable crystal structure,
specifically, those materials having a layered structure such as
lithium cobalt oxide and lithium nickel cobalt manganese oxide,
those materials having a spinel structure such as lithium manganese
oxide and lithium nickel manganese oxide, and those materials
having an olivine structure. The positive electrode active material
may have a single particle size or may be a mixture of particles
with different sizes.
[Negative Electrodes]
[0059] For example, the negative electrode is composed of a
negative electrode current collector such as a metallic foil, and a
negative electrode mixture layer disposed on the current collector.
The negative electrode current collector may be, for example, a
foil of a metal that is stable at negative electrode potentials
such as copper, or a film having a skin layer of such a metal. The
negative electrode mixture layer includes a negative electrode
active material, and preferably further includes a binder. The
negative electrode may be fabricated by, for example, applying a
negative electrode mixture slurry including the negative electrode
active material and other components such as a binder onto a
negative electrode current collector, and drying and rolling the
wet films so as to form negative electrode mixture layers on both
sides of the current collector.
[0060] The negative electrode active material is not particularly
limited as long as it can reversibly store and release lithium
ions. Examples include carbon materials such as natural graphite
and artificial graphite, metals which can be alloyed with lithium
such as silicon (Si) and tin (Sn), and alloys and composite oxides
containing metal elements such as Si and Sn. The negative electrode
active materials may be used singly, or two or more may be used in
combination.
[0061] Examples of the binders include, similarly to those in the
positive electrodes, fluororesins, PAN, polyimide resins, acrylic
resins and polyolefin resins. When the mixture slurry is prepared
using an aqueous solvent, it is preferable to use, among others,
CMC or a salt thereof (such as CMC-Na, CMC-K or CMC-NH.sub.4, or a
partially neutralized salt), styrene-butadiene rubber (SBR),
polyacrylic acid (PAA) or a salt thereof (such as PAA-Na or PAA-K,
or a partially neutralized salt), or polyvinyl alcohol (PVA).
[Separators]
[0062] As the separator, a porous sheet having ion permeability and
insulating properties is used. Specific examples of the porous
sheets include microporous thin films, woven fabrics and nonwoven
fabrics. Some preferred materials of the separators are polyolefin
resins such as polyethylene and polypropylene, and celluloses. The
separator may be a stack having a cellulose fiber layer and a
thermoplastic resin fiber layer such as of a polyolefin resin.
Alternatively, the separator may be a multilayered separator
including a polyethylene layer and a polypropylene layer, or may be
one having a coating of an aramid resin or the like on the surface
of the separator.
[0063] A filler layer including an inorganic filler may be disposed
in the interface of the separator and at least one of the positive
electrode and the negative electrode. Examples of the inorganic
fillers include oxides containing at least one of titanium (Ti),
aluminum (Al), silicon (Si) and magnesium (Mg), and phosphoric acid
compounds containing at least one of titanium (Ti), aluminum (Al),
silicon (Si) and magnesium (Mg), wherein the surface of these
compounds may be treated with hydroxides or the like. For example,
the filler layer may be formed by applying a slurry containing the
filler onto the surface of the positive electrode, the negative
electrode or the separator.
[Nonaqueous Electrolytes]
[0064] The nonaqueous electrolyte includes a nonaqueous solvent and
a solute dissolved in the nonaqueous solvent. Examples of the
nonaqueous solvents include esters, ethers, nitriles, amides such
as dimethylformamide, isocyanates such as
hexamethylenediisocyanate, and mixed solvents including two or more
of these solvents. The nonaqueous solvent may include a halogenated
compound resulting from the substitution of any of the above
solvents with halogen atoms such as fluorine in place of at least
part of the hydrogen atoms.
[0065] Examples of the esters include cyclic carbonate esters such
as ethylene carbonate (EC), propylene carbonate (PC) and butylene
carbonate, chain carbonate esters such as dimethyl carbonate (DMC),
methyl ethyl carbonate (EMC), diethyl carbonate (DEC), methyl
propyl carbonate, ethyl propyl carbonate and methyl isopropyl
carbonate, cyclic carboxylate esters such as .gamma.-butyrolactone
and .gamma.-valerolactone, and chain carboxylate esters such as
methyl acetate, ethyl acetate, propyl acetate, methyl propionate
(MP) and ethyl propionate.
[0066] Examples of the ethers include cyclic ethers such as
1,3-dioxolane, 4-methyl-1,3-dioxolane, tetrahydrofuran,
2-methyltetrahydrofuran, propylene oxide, 1,2-butylene oxide,
1,3-dioxane, 1,4-dioxane, 1,3,5-trioxane, furan, 2-methylfuran,
1,8-cineole and crown ethers, and chain ethers such as
1,2-dimethoxyethane, diethyl ether, dipropyl ether, diisopropyl
ether, dibutyl ether, dihexyl ether, ethyl vinyl ether, butyl vinyl
ether, methyl phenyl ether, ethyl phenyl ether, butyl phenyl ether,
pentyl phenyl ether, methoxytoluene, benzyl ethyl ether, diphenyl
ether, dibenzyl ether, o-dimethoxybenzene, 1,2-diethoxyethane,
1,2-dibutoxyethane, diethylene glycol dimethyl ether, diethylene
glycol diethyl ether, diethylene glycol dibutyl ether,
1,1-dimethoxymethane, 1,1-diethoxyethane, triethylene glycol
dimethyl ether and tetraethylene glycol dimethyl.
[0067] Examples of the nitriles include acetonitrile,
propionitrile, butyronitrile, valeronitrile, n-heptanitrile,
succinonitrile, glutaronitrile, adiponitrile, pimelonitrile,
1,2,3-propanetricarbonitrile and 1,3,5-pentanetricarbonitrile.
[0068] Some preferred halogenated compounds are fluorinated cyclic
carbonate esters such as fluoroethylene carbonate (FEC),
fluorinated chain carbonate esters, and fluorinated chain
carboxylate esters such as fluoromethyl propionate (FMP).
[0069] The solute may be any known solute that is conventionally
used. Examples include fluorine-containing lithium salts such as
LiPF.sub.6, LiBF.sub.4, LiCF.sub.3SO.sub.3, LiN(FSO.sub.2).sub.2,
LiN(CF.sub.3SO.sub.2).sub.2, LiN(C.sub.2F.sub.5SO.sub.2).sub.2,
LiN(CF.sub.3SO.sub.2)(C.sub.4F.sub.9SO.sub.2),
LiC(C.sub.2F.sub.5SO.sub.2).sub.3 and LiAsF.sub.6. Further, a
lithium salt other than fluorine-containing lithium salts [a
lithium salt containing one or more elements of P, B, O, S, N and
Cl (such as, for example, LiClO.sub.4)] may be added to the
fluorine-containing lithium salt. In particular, it is preferable
that the solute include a fluorine-containing lithium salt and a
lithium salt having an oxalato complex as the anion because such a
solute forms a film on the negative electrode surface which is
stable even under high-temperature conditions.
[0070] Examples of the lithium salts having an oxalato complex as
the anion include LiBOB [lithium-bisoxalatoborate],
Li[B(C.sub.2O.sub.4)F.sub.2], Li[P(C.sub.2O.sub.4)F.sub.4] and
Li[P(C.sub.2O.sub.4).sub.2F.sub.2]. In particular, it is preferable
to use LiBOB, which can form a very stable film on the negative
electrode. The solutes may be used singly, or two or more may be
used in combination.
[0071] An overcharge inhibitor may be added to the nonaqueous
electrolyte. For example, cyclohexylbenzene (CHB) may be used.
Further, use may be made of benzene derivatives such as benzene,
biphenyl, alkylbiphenyls, for example, 2-methylbiphenyl, terphenyl,
partially hydrogenated terphenyl, naphthalene, toluene, anisole,
cyclopentylbenzene, t-butylbenzene and t-amylbenzene, phenyl ether
derivatives such as phenyl propionate and 3-phenylpropyl acetate,
and halides of these compounds. These compounds may be used singly,
or two or more may be used in combination.
EXPERIMENTAL EXAMPLES
[0072] Hereinbelow, the present disclosure will be described in
greater detail based on experimental examples. The scope of the
present disclosure is not limited to such experimental
examples.
First Experimental Examples
Experimental Example 1
[Preparation of Positive Electrode Active Material]
[0073] LiOH and an oxide obtained by heat treating coprecipitated
nickel cobalt aluminum composite hydroxide represented by
Ni.sub.0.91Co.sub.0.06Al.sub.0.03(OH).sub.2 at 500.degree. C. were
mixed together using an Ishikawa-type grinder mortar in a molar
ratio of Li to the transition metals of 1.05:1. Next, the resultant
mixture was heat treated at 760.degree. C. for 20 hours in an
oxygen atmosphere and was thereafter crushed. As a result,
particles of lithium nickel cobalt aluminum composite oxide
(lithium transition metal oxide) represented by
Li.sub.1.05Ni.sub.0.91Co.sub.0.06Al.sub.0.03O.sub.2 which had an
average secondary particle size of about 11 .mu.m were
obtained.
[0074] 1000 g of the lithium transition metal oxide particles were
provided. The particles were added to 1.5 L of pure water and the
mixture was stirred to give a suspension of the lithium transition
metal oxide dispersed in pure water. Next, a 0.1 mol/L aqueous
erbium sulfate solution obtained by dissolving erbium oxide into
sulfuric acid, and a 1.0 mol/L aqueous magnesium sulfate solution
were added in several portions to the suspension. During the
addition of the aqueous erbium sulfate solution to the suspension,
the pH of the suspension was 11.5 to 12.0. Next, the suspension was
filtered, and the powder obtained was washed with pure water, dried
in vacuum at 200.degree. C. and heat treated in an oxygen
atmosphere at 600.degree. C. A positive electrode active material
was thus prepared. The median particle size (D50, volume-basis) of
the positive electrode active material particles was about 10 .mu.m
(measured with LA920 manufactured by HORIBA, Ltd.).
[0075] The surface of the positive electrode active material
obtained was observed with SEM. The observation confirmed that
primary particles of the erbium compound having an average particle
size of 20 to 30 nm had been aggregated into secondary particles of
the erbium compound with an average particle size of 100 to 200 nm,
and the secondary particles had been attached to the surface of
secondary particles of the lithium transition metal oxide. The
observation also confirmed that most of the secondary particles of
the erbium compound had been attached to the recesses formed
between the primary particles of the lithium transition metal oxide
that were adjacent to one another on the surface of the secondary
particles of the lithium transition metal oxide, and that the
secondary particles that had been attached were in contact with
both of the primary particles adjacent to each other in the recess.
The amount of the erbium compound attached was measured by ICP
emission spectroscopy to be 0.15 mass % in terms of erbium element
relative to the lithium nickel cobalt aluminum composite oxide.
[0076] Deposits that seemed to be the magnesium compound were
substantially absent on the surface of the secondary particles of
the lithium transition metal oxide. EDS elemental mapping of a
cross section of the secondary particle showed that Mg was present
in the inside of the primary particles of the lithium transition
metal oxide. Mg was particularly enriched in the region from the
surface to a depth of 2 .mu.m of the secondary particle. The
particle size of the secondary particle (the diameter of a circle
circumscribed on the particle in the SEM image) was about 10 .mu.m.
The Mg concentration was measured by ICP emission spectroscopy to
be 0.1 mol % relative to the total molar amount of metal element(s)
except Li. From the depth up to which Mg was found by the elemental
mapping and the Mg concentration measured by ICP emission
spectroscopy, the Mg concentration in the skin region of the
secondary particle (the region from the surface to a depth of 2
.mu.m of the secondary particle) was calculated to be 0.17 mol
%.
[0077] Because the pH of the suspension in EXPERIMENTAL EXAMPLE 1
was high at 11.5 to 12.0, it is probable that the primary particles
of erbium hydroxide precipitated in the suspension bonded
(aggregated) to one another into secondary particles. Further,
because the proportion of Ni in EXPERIMENTAL EXAMPLE 1 was as high
as 91% and consequently the proportion of trivalent Ni was high,
the proton exchange between LiNiO.sub.2 and H.sub.2O was
facilitated to occur at interfaces of the primary particles of the
lithium transition metal oxide, and a large amount of LiOH
generated by the proton exchange reaction came out of the inside of
the interfaces of the adjacent primary particles exposed on the
surface of the secondary particles of the lithium transition metal
oxide. As a result, the alkali concentration between the primary
particles adjacent to one another on the surface of the lithium
transition metal oxide was increased. It is therefore probable that
the erbium hydroxide particles were precipitated in the suspension
and formed secondary particles by being aggregated to the recesses
present at interfaces of the primary particles just like the
particles were attracted by the alkali.
[0078] The precipitation of magnesium does not respond to alkali
concentration as sharply as erbium, and thus magnesium tends to be
precipitated uniformly over the surface of the secondary particles
of the lithium transition metal oxide. SEM observation of the
particles before heat treatment confirmed that the magnesium
compound had been precipitated uniformly on the surface of the
secondary particles. In contrast, most of the magnesium compound
uniformly precipitated on the surface of the secondary particles
was not seen on the surface in the observation of the particles
after heat treatment (calcination). This probably shows that most
of Mg was diffused and dissolved into the inside of the
particles.
[Fabrication of Positive Electrode]
[0079] Carbon black and an N-methyl-2-pyrrolidone solution of
polyvinylidene fluoride were weighed in such amounts that the mass
ratio of the positive electrode active material particles to the
conductive agent and the binder would be 100:1:1. These components
and the positive electrode active material particles were kneaded
together with use of T. K. HIVIS MIX (manufactured by PRIMIX
Corporation) to give a positive electrode mixture slurry.
[0080] Next, the positive electrode mixture slurry was applied to
both sides of a positive electrode current collector composed of an
aluminum foil, and the wet films were dried and rolled with a
roller. A current collector tab made of aluminum was connected to
the current collector. A positive electrode plate was thus
fabricated which had the positive electrode mixture layers on both
sides of the positive electrode current collector. The packing
density of the positive electrode active material in the positive
electrode was 3.60 g/cm.sup.3.
[Fabrication of Negative Electrode]
[0081] Artificial graphite as a negative electrode active material,
CMC (carboxymethylcellulose sodium) and SBR (styrene-butadiene
rubber) were mixed together in a mass ratio of 100:1:1 in an
aqueous solution to give a negative electrode mixture slurry. Next,
the negative electrode mixture slurry was applied uniformly to both
sides of a negative electrode current collector composed of a
copper foil, and the wet films were dried and rolled with a roller.
A current collector tab made of nickel was connected to the current
collector. A negative electrode plate was thus fabricated which had
the negative electrode mixture layers on both sides of the negative
electrode current collector. The packing density of the negative
electrode active material in the negative electrode was 1.75
g/cm.sup.3.
[Preparation of Nonaqueous Electrolytic Solution]
[0082] Ethylene carbonate (EC), methyl ethyl carbonate (MEC) and
dimethyl carbonate (DMC) were mixed in a volume ratio of 2:2:6.
Lithium hexafluorophosphate (LiPF.sub.6) was dissolved into the
mixed solvent so that its concentration would be 1.3 mol/L, and
thereafter vinylene carbonate (VC) was dissolved into the mixed
solvent so that its concentration would be 2.0 mass %.
[Fabrication of Battery]
[0083] The positive electrode and the negative electrode obtained
above were wound into a coil via a separator between the
electrodes. The winding core was pulled out, and a wound electrode
assembly was obtained. Next, the wound electrode assembly was
pressed into a flat electrode assembly. Thereafter, the flat
electrode assembly and the nonaqueous electrolytic solution were
inserted into an exterior case made of an aluminum laminate.
Battery A1 was thus fabricated. The size of the battery was 3.6 mm
in thickness, 35 mm in width and 62 mm in length. The nonaqueous
electrolyte secondary battery was charged to 4.20 V and discharged
to 3.0 V, and the discharge capacity during this process was 950
mAh.
Experimental Example 2
[0084] Battery A2 was fabricated in the same manner as in
EXPERIMENTAL EXAMPLE 1, except that the aqueous magnesium sulfate
solution was not added during the preparation of the positive
electrode active material.
Experimental Example 3
[0085] A positive electrode active material was prepared and
Battery A3 was fabricated using the positive electrode active
material in the same manner as in EXPERIMENTAL EXAMPLE 1, except
that in the preparation of the positive electrode active material,
the pH of the suspension was kept constant at 9 during the addition
of the aqueous erbium sulfate solution to the suspension. The
suspension was controlled to pH 9 by appropriate addition of a 10
mass % aqueous sodium hydroxide solution.
[0086] The surface of the positive electrode active material
obtained was observed by SEM. The observation showed that the
primary particles of erbium hydroxide having an average particle
size of 10 nm to 50 nm did not form secondary particles and had
been uniformly dispersed and attached as such to the entire surface
(to elevated portions and to recesses) of the secondary particles
of the lithium transition metal oxide. The amount of the erbium
compound attached was measured by ICP emission spectroscopy to be
0.15 mass % in terms of erbium element relative to the lithium
nickel cobalt aluminum composite oxide.
[0087] It is probable that in EXPERIMENTAL EXAMPLE 3, particles of
erbium hydroxide were precipitated at a lowered rate in the
suspension due to the pH of the suspension being 9 and consequently
the erbium hydroxide particles, without forming secondary
particles, were precipitated uniformly over the entire surface of
the secondary particles of the lithium transition metal oxide.
Experimental Example 4
[0088] Battery A4 was fabricated in the same manner as in
EXPERIMENTAL EXAMPLE 3, except that the aqueous magnesium sulfate
solution was not added during the preparation of the positive
electrode active material.
Experimental Example 5
[0089] A positive electrode active material was prepared and
Battery A5 was fabricated using the positive electrode active
material in the same manner as in EXPERIMENTAL EXAMPLE 1, except
that in the preparation of the positive electrode active material,
the aqueous erbium sulfate solution was not added and thus no
erbium hydroxide was attached to the surface of the secondary
particles of the lithium transition metal oxide.
Experimental Example 6
[0090] Battery A6 was fabricated in the same manner as in
EXPERIMENTAL EXAMPLE 5, except that the aqueous magnesium sulfate
solution was not added during the preparation of the positive
electrode active material.
Experimental Example 7
[0091] Battery A7 was fabricated in the same manner as in
EXPERIMENTAL EXAMPLE 1, except that in the preparation of the
positive electrode active material, the Mg content was controlled
to 0.03 mol % relative to the total molar amount of metal
element(s) except Li in the lithium transition metal oxide. The Mg
concentration in the skin region of the secondary particle (the
region from the surface to a depth of 2 .mu.m of the secondary
particle) was measured in the same manner as in EXPERIMENTAL
EXAMPLE 1, and the Mg concentration was determined to be 0.05 mol
%.
[Measurement of DCR]
[0092] The batteries were each tested under the following
conditions to measure DCR before charge discharge cycles and after
100 cycles.
<Measurement of DCR Before Cycles>
[0093] The battery was charged at a current of 475 mA to 100% SOC.
While keeping the battery voltage constant at the voltage at which
SOC (state of charge) had reached 100%, the battery was charged
until the current value reached 30 mA. After the completion of
charging, the battery was allowed to rest for 120 minutes and the
open circuit voltage (OCV) was measured. The battery was then
discharged at 475 mA for 10 seconds, and the voltage after 10
seconds of discharging was measured. DCR before cycles (100% SOC)
was calculated using the following equation (1).
DCR (.OMEGA.)=(OCV (V) after 120 minutes of rest-Voltage (V) after
10 seconds of discharging)/(Current value (A)) (1)
[0094] Thereafter, a cycle of charging and discharging under the
following conditions was repeated 150 times. The interval of time
between the measurement of DCR before cycles and the charge
discharge cycle test was 10 minutes.
<Charge Discharge Cycle Test>
[0095] Charging Conditions
[0096] The battery was charged at a constant current of 475 mA
until the battery voltage reached 4.2 V (the positive electrode
potential reached 4.3 V versus lithium). After the battery voltage
had reached 4.2 V, the battery was charged at a constant voltage of
4.2 V until the current value reached 30 mA.
[0097] Discharging Conditions
[0098] The battery was discharged at a constant current of 950 mA
until the battery voltage reached 3.0 V.
[0099] Rest Conditions
[0100] The interval of time between the charging and the
discharging was 10 minutes.
<Measurement of DCR after 150 Cycles>
[0101] The value of DCR after 150 cycles was measured in the same
manner as DCR had been measured before the cycles. The interval of
time between the charge discharge cycle test and the measurement of
DCR after cycles was 10 minutes. The measurement of DCR and the
charge discharge cycle test were both carried out in a thermostatic
chamber at 45.degree. C.
[Calculation of DCR Increase Ratio]
[0102] The ratio of the increase in DCR after 150 cycles was
calculated using the following equation (2). The results are
described in Table 1.
DCR increase ratio (100% SOC)=(DCR after 150 cycles (100%
SOC))/(DCR before cycles (100% SOC)).times.100 (2)
TABLE-US-00001 TABLE 1 Rare Manner Amount of DCR earth in which
rare earth dissolved Mg increase Battery element compound was
attached (mol %) ratio (%) A1 Er Aggregated in recesses 0.1 37 A2
Er Aggregated in recesses 0 44 A3 Er Uniformly dispersed 0.1 50 A4
Er Uniformly dispersed 0 51 A5 None -- 0.1 48 A6 None -- 0 49 A7 Er
Aggregated in recesses 0.03 38
[0103] Battery A1 will be discussed below. In the positive
electrode active material of Battery A1, the secondary particles of
the rare earth compound were attached to both primary particles of
the lithium transition metal oxide that were adjacent to each other
in the recesses (see FIG. 3). Because of this, the surface of every
primary particles was probably prevented from surface alteration
and breakage at interfaces of the primary particles during the
charge discharge cycles. In addition, the secondary particles of
the rare earth compound has an effect of fixing (bonding) together
the primary particles that constitute the lithium transition metal
oxide. Because of this, the primary particles were probably
prevented from breakage at their interfaces in the recesses in the
lithium transition metal oxide.
[0104] Further, the positive electrode active material of Battery
A1 contained Mg dissolved in the lithium transition metal oxide.
Because of this, the primary particles present inside the particles
of the lithium transition metal oxide were probably prevented from
alteration and breakage at their interfaces.
[0105] In Battery A1, the positive electrode active material was
prevented from surface alteration and breakage both on the surface
and in the inside of the positive electrode active material.
Probably because of this, the ratio of DCR increase after charge
discharge cycles was reduced. Battery A1 attained a marked
enhancement in the reduction of the DCR increase ratio by the
synergetic effect of the secondary particles of the rare earth
compound attached to the recesses in the lithium transition metal
oxide, in combination with Mg dissolved in the lithium transition
metal oxide.
[0106] Batteries A3 and A5 will be discussed below. As illustrated
in FIG. 4, the positive electrode active material used in Battery
A3 was such that the primary particles 24 of the rare earth
compound were attached, without forming secondary particles,
uniformly over the entire surface of the secondary particles 21 of
the lithium transition metal oxide. In the positive electrode
active material used in Battery A5, as illustrated in FIG. 5, no
rare earth compounds were attached to the surface of the secondary
particles 21 of the lithium transition metal oxide.
[0107] In Batteries A3 and A5, the recesses 23 in the lithium
transition metal oxide were free from attachment of secondary
particles of the rare earth compound. Because of this, it was
probably impossible to prevent surface alteration of the primary
particles 20 forming the recesses 23, and to prevent breakage at
interfaces of the primary particles. Although the positive
electrode active materials of Batteries A3 and A5 contained
dissolved Mg, the increase in DCR is more greatly affected by the
deterioration of the surface of the secondary particles than by the
deterioration of the inside of the secondary particles. Probably
because of this, these batteries showed a higher DCR increase ratio
than Battery A1.
[0108] Batteries A2, A4 and A6 will be discussed. The positive
electrode active materials of Batteries A2, A4 and A6 were
different from the positive electrode active materials of Batteries
A1, A3 and A5, respectively, in that Mg was not dissolved
therein.
[0109] In Battery A2, the secondary particles of the rare earth
compound were attached to both primary particles that were adjacent
to each other in the recesses in the lithium transition metal
oxide. Because of this, for the same reasons as described above in
connection with Battery A1, the surface of every primary particles
forming the recesses was probably prevented from surface alteration
and breakage at interfaces of the primary particles. In Battery A2,
however, the absence of dissolved Mg in the positive electrode
active material resulted in a failure to suppress deterioration and
breakage at interfaces of the primary particles in the inside of
the secondary particles, in particular, near the surface of the
secondary particles. Probably because of this, Battery A2 suffered
an increase in positive electrode resistance and showed a higher
DCR increase ratio than Battery A1.
[0110] In Batteries A4 and A6, there were no secondary particles of
the rare earth compound attached to the recesses in the lithium
transition metal oxide, and consequently it was impossible to
suppress surface alteration of the primary particles 20 forming the
recesses, and to suppress breakage at interfaces of the primary
particles. In addition, Batteries A4 and A6 did not have Mg
dissolved in the positive electrode active material. These absences
resulted in a failure to suppress alteration and breakage at
interfaces of the primary particles both on the surface and in the
inside of the secondary particles. Probably because of this,
Batteries A4 and A6 suffered a larger increase in positive
electrode resistance than Battery A2 and showed a still higher
ratio of DCR increase after charge discharge cycles.
[0111] Mg was used in Battery A7 and effectively prevented
alteration and breakage at interfaces of the primary particles in
the inside of the particles, although the amount thereof was
smaller than in Battery A1 and thus the suppressive effect was
smaller than that obtained in Battery A1. Probably because of this,
the DCR increase ratio was substantially equal to that of Battery
A1.
Second Experimental Examples
[0112] While FIRST EXPERIMENTAL EXAMPLES involved erbium as the
rare earth element, SECOND EXPERIMENTAL EXAMPLES studied batteries
using samarium or neodymium as the rare earth element.
Experimental Example 8
[0113] A positive electrode active material was prepared and
Battery A8 was fabricated using the positive electrode active
material in the same manner as in EXPERIMENTAL EXAMPLE 1, except
that in the preparation of the positive electrode active material,
the aqueous erbium sulfate solution was replaced by an aqueous
samarium sulfate solution. The amount of the samarium compound
attached was measured by ICP emission spectroscopy to be 0.13 mass
% in terms of samarium element relative to the lithium nickel
cobalt aluminum composite oxide. The Mg concentration in the skin
region of the secondary particle (the region from the surface to a
depth of 2 .mu.m of the secondary particle) was measured in the
same manner as in EXPERIMENTAL EXAMPLE 1, and the Mg concentration
was determined to be 0.17 mol %.
Experimental Example 9
[0114] A positive electrode active material was prepared and
Battery A9 was fabricated using the positive electrode active
material in the same manner as in EXPERIMENTAL EXAMPLE 1, except
that in the preparation of the positive electrode active material,
the aqueous erbium sulfate solution was replaced by a neodymium
sulfate solution. The amount of the neodymium compound attached was
measured by ICP emission spectroscopy to be 0.13 mass % in terms of
neodymium element relative to the lithium nickel cobalt aluminum
composite oxide. The Mg concentration in the skin region of the
secondary particle (the region from the surface to a depth of 2
.mu.m of the secondary particle) was measured in the same manner as
in EXPERIMENTAL EXAMPLE 1, and the Mg concentration was determined
to be 0.17 mol %.
[0115] With respect to Batteries A8 and A9, the ratio of DCR
increase after 150 cycles was calculated under the same conditions
as in FIRST EXPERIMENTAL EXAMPLES.
TABLE-US-00002 TABLE 2 Rare Manner Amount of DCR earth in which
rare earth dissolved Mg increase Battery element compound was
attached (mol %) ratio (%) A1 Er Aggregated in recesses 0.1 37 A8
Sm Aggregated in recesses 0.1 39 A9 Nd Aggregated in recesses 0.1
38
[0116] As apparent from Table 2, the DCR increase ratio is reduced
also when erbium is replaced by samarium or neodymium that
similarly belongs to the rare earth elements. It is therefore
believed that the DCR increase ratio will be reduced similarly even
when a rare earth element other than erbium, samarium and neodymium
is used.
INDUSTRIAL APPLICABILITY
[0117] The present invention can be applied to nonaqueous
electrolyte secondary batteries.
REFERENCE SIGNS LIST
[0118] 1 positive electrode [0119] 2 negative electrode [0120] 3
separator [0121] 4 positive electrode current collector tab [0122]
5 negative electrode current collector tab [0123] 6 aluminum
laminate case [0124] 7 closed portion [0125] 11 nonaqueous
electrolyte secondary battery [0126] 20 primary particle of lithium
transition metal oxide (primary particle) [0127] 21 secondary
particle of lithium transition metal oxide (secondary particle)
[0128] 23 recess [0129] 24 primary particle of rare earth compound
(primary particle) [0130] 25 secondary particle of rare earth
compound (secondary particle)
* * * * *